Such tanks are used, in particular in the nuclear power industry, as storage tanks for storing liquids such as de-gassed distilled water.
Very high reliability is required in this industry in the design of tanks, in order to eliminate any risk of operation being stopped, or worse still any deterioration of their essential component parts. It is also essential for operation to be safe without there being any risk of operating error. Furthermore, since such floating roof tanks are used for topping up the primary circuits of a nuclear reactor, it is necessary for the storage to be protected from the air so as to ensure that the stored liquid is of very high quality, for example, in this particular application de-gased water must have an oxygen content of no more than 100 .mu.g/l in order to avoid any risk of oxidation.
Conventionally, such floating roof tanks comprise a cylindrical wall and a floating roof which is surrounded by said wall, and which is connected to it in sealed manner by means of a flexible membrane. In order to preserve this flexible membrane connecting the periphery of the roof to the side wall of the tank, and in particular in order to avoid kinks forming therein and/or an uncontrolled application thereof against the wall of the tank under the effects of the pressure of the liquid contained in the tank, it is common practice for the space defined by the membrane and the wall of the tank, and in communication with the outside, to receive a counter-pressure liquid which acts essentially as a lubricant for the membrane when the roof moves, and which additionally, at least in some positions, serves to center the said roof relative to the cylindrical wall of the tank.
One of the problems in designing such flotating roof tanks lies in protecting them against being over-filled when the roof is in the top position. Following an erroneous operation it may happen that an attempt is made to over-fill the tank. In general, upward motion of the roof is limited by top stops provided at the top of the tank, e.g. around its wall, and if the pressure continues to rise within the tank while the roof is in abutment against its top stops, it is essential for means to be provided to protect the tank.
One known technique, as described, for example, in French patent specification No. 2526405, is to provide a valve on the roof, sometimes referred to as an overflow valve, in order to prevent the pressure from rising too high inside the tank, by allowing the entire input flowrate to escape beyond some threshold pressure. Such techniques are not satisfactory since the liquid passing through the valve during such a period of excess pressure flows directly into the roof caisson, which may cause it to sink and lead to many kinds of risk as it moves downwardly in a disorderly manner. Emptying the roof via the inside of the tank would be very difficult and complicated to provide since a high duct runs the danger of catching in the membrane, while a low duct would require a flexible length of pipework connected to the roof yet subjected on the outside to the pressure of the liquid inside the tank, i.e. to a pressure of about 1 bar in a tank which is about 10 m deep when the roof is in the high position.
Another known technique consists in providing a floating roof with a rigid outer belt sliding like a piston ring against the inside wall of the tank, with said belt unmasking an exit orifice when the roof is in its top position. Such a structure, for example as described in U.S. Pat. No. 2,712,395, is incompatible with using a volume of counter-pressure liquid received in a peripheral space defined by a membrane connecting the periphery of the roof to the side wall of the tank.
Outside the field of floating roof tanks, there exist other types of safety device for liquid tanks, for example, proposals have been made to use a compensation vessel which is separate from the tank. One such device is described in French patent specification No. 841839: the vessel contains a compensating liquid which is denser and immiscible with the stored liquid, and whose level is adjusted so as to automatically maintain a fixed pressure against the fixed roof of the tank. Such a device would not be appropriate for a floating roof tank having a volume of counter-pressure liquid, since it is used for very high overpressures. The present invention aims to avoid the above-mentioned drawbacks of the prior art.
One aim of the invention is thus to provide a floating roof tank whose structure makes it possible not only to provide safe protection against overpressure due to over-filling the tank with the roof is in its top position, but also makes it possible to remove excess liquid effectively in such an overfilled situation.
Another aim of the invention is to provide a structure for the tank which is both simple and reasonably cheap to manufacture.
Another aim of the invention is to preserve as much as possible of the liquid stored in the tank from contact with ambient air, which is particularly useful in nuclear power industry applications.